gh-110205: Fix asyncio ThreadedChildWatcher._join_threads() (#110884)

- `ThreadedChildWatcher.close()` is now *officially* a no-op; `_join_threads()` never did anything.
- Threads created by that class are now named `asyncio-waitpid-NNN`.
- `test.test_asyncio.utils.TestCase.close_loop()` now waits for the child watcher's threads, but not forever; if a thread hangs, it raises `RuntimeError`.

Lib/asyncio/unix_events.py

@@ -1371,14 +1371,7 @@ class ThreadedChildWatcher(AbstractChildWatcher):
         return True
 
     def close(self):
-        self._join_threads()
-
-    def _join_threads(self):
-        """Internal: Join all non-daemon threads"""
-        threads = [thread for thread in list(self._threads.values())
-                   if thread.is_alive() and not thread.daemon]
-        for thread in threads:
-            thread.join()
+        pass
 
     def __enter__(self):
         return self
@@ -1397,7 +1390,7 @@ class ThreadedChildWatcher(AbstractChildWatcher):
     def add_child_handler(self, pid, callback, *args):
         loop = events.get_running_loop()
         thread = threading.Thread(target=self._do_waitpid,
-                                  name=f"waitpid-{next(self._pid_counter)}",
+                                  name=f"asyncio-waitpid-{next(self._pid_counter)}",
                                   args=(loop, pid, callback, args),
                                   daemon=True)
         self._threads[pid] = thread